1. Field of the Invention
The present invention relates to an optical link amplifier which is employed in an optical communication network, especially an optical link amplifier which has a large insertion loss such as a semiconductor laser amplifier, and a wavelength multiplex oscillator, more particularly a wavelength multiplex laser oscillator which is a component used for wavelength multiplex transmission in an optical communication.
2. Description of the Related Art
There is a growing trend in recent years to employ local area networks (LANs) as data communication networks among relatively closely installed computers, workstations, etc. One of such LANs is Ethernet.RTM. developed by Xerox Corporation.
FIG. 1 is a diagram showing an Ethernet.RTM.-based network. In FIG. 1, reference numerals designate: a coaxial cable; 2, taps (branches); and 3, nodes (terminals or stations). Each node 3 is connected to the coaxial cable 1 through the tap 2. To increase nodes 3, taps 2 must be additionally provided for connection to additional nodes 3.
Meanwhile, there is a conspicuous development in optical communications using optical fibers as transmission media. The optical fiber, because of its large bandwidth, low loss, high noise resistance, makes itself suitable to LANs. However, since the optical fiber cannot provide taps as with the coaxial cable, it is difficult to configure a network similar to Ethernet.RTM..
To overcome this problem, a network configuration has been proposed, in which terminals are provided separately for transmission and reception per node and a star coupler is employed to distribute a signal to all such nodes. See, e.g., G. Rawson, IEEE Transactions on Communications, Vol. COM-26, No. 7, July 1978, "Fibernet: Multimode Optical Fibers for Local Computer Networks."
FIG. 2 shows the proposed example of an optical communication network using a star coupler. In FIG. 2, reference numerals 4a and 4b designate optical fibers; 5, a star coupler of a mixing rod type; and 6, terminals.
A signal from each node 3 is converted into an optical signal by its corresponding light-emitting element 7 to be supplied to the star coupler 5 through its corresponding optical fiber 4a. These optical signals are all mixed all together by the star coupler 5 and then distributed to the light-receiving elements 8 through the respective optical fibers 4b, and reconverted into electric signals. The electric signals are supplied to the respective nodes 3.
Accordingly, the signal transmitted from a single Rode is transmitted to all the nodes (multiple access), thus allowing a communication network similar to Ethernet.RTM. to be implemented.
However, an optical communication network using such a star coupler has the shortcoming that the expandability of a network is limited by the number of terminals of the star coupler. This is because interconnection between star couplers results in forming a closed loop within the transmission path and this causes phenomena such as oscillation and attenuating vibration.
To avoid this problem, the network may be configured, as disclosed in the specification of Unexamined Japanese Patent Publication No. Hei. 3-296332, so that the transfer constant between the input and output terminals forming a pair can be made zero, the pair of the terminals being connected to a single node from the star coupler. This allows a plurality of star couplers to be interconnected from one to another for network expansion. Also, the network thus constructed permits bidirectional communication between two nodes, and this bidirectional communication facilitates contention detection. Further, as proposed in the co-pending U.S. patent application Ser. No. 07/813,443 filed Dec. 26, 1991, the disclosure of which is incorporated herein by reference, the number of optical fibers, optical connectors, and link amplifiers can be almost halved by making a single terminal support the function of inputting and outputting, instead of arranging an input terminal and an output terminal separately.
By the way, a transmission control procedure for transmitting signals (hereinafter referred to as "protocol") is generally specified for each communication network. The protocol adopted by Ethernet.RTM. is of a contention system.
In a signal transmission conforming to a contention protocol in the FIG. 1 network, it is checked that there is a signal transmitted from another node on the bus, i.e., the coaxial cable 1 to transmit a signal from a node 3. If no signal is present on the bus, the node initiates transmission, and if a signal is present, the node does not transmit but waits. Therefore, the bus is used on a first-come-first-served basis. It is for this reason that this type of protocol is called a contention protocol. The protocol employed by Ethernet.RTM. is called CSMA/CD (Carrier Sense Multiple Access/Collision Detection), and is based on the above control procedure.
This contention protocol allows the bus to be efficiently used in terms of time, making itself suitable to data communication. However, LANs that adopt the contention protocol are not suitable for transmission of signals such as audio and video signals requiring realtime operation. These audio and video signals, etc., once their transmission has been initiated, must be transmitted by a predetermined amount (e.g., in the order of several K bits) at least before a predetermined short time (e.g., in the order of a millisecond) elapses to provide smooth sound and images. However, once transmission has been started, it is likely that the bus will be occupied by another node during transmission under this contention protocol. The contention protocol may ensure the realtime operation as long as the bus is not so busy, but this is the matter of probability, and it is a problem which must be taken care of to ensure stable communication. An optical network system, which has a long optical signal path, must have disposed an optical amplifier somewhere along the path to make up the attenuation of the optical signal. FIG. 15 shows an example of an ordinary optical communication network using optical amplifiers. A plurality of terminal units 3a, 3b, 3c are connected to an optical transmission path 1 made of, e.g., an optical fiber for transmitting an optical signal through couplers 9a, 9b, 9c, respectively, and optical link amplifiers 30a, 30b for compensating for optical signal attenuation are inserted at predetermined points along the optical transmission path 1.
Typical optical amplifiers used as optical link amplifiers include: a semiconductor laser optical amplifier and a rare earth element-doped optical fiber amplifier.
Semiconductor laser optical amplifiers use an ordinary semiconductor laser as an optical amplifier element, or operate a semiconductor laser as a traveling-wave optical amplifier by applying anti-reflecting films on both faces thereof (see, e.g., Mochizuki: "Semiconductor Laser Optical Amplifiers", kogaku, vol. 18, No. 6, (June 1989), pp. 297-302). Rare earth element-doped optical fiber amplifiers irradiate an excited beam from an exciting beam source such as a laser onto the optical fiber into which a rare earth element has been doped, and thus amplifies an input beam by keeping the optical fiber excited (see. e.g, Horiguchi: "Optical Fiber Amplifiers", kogaku, vol. 19, No. 5 (May 1990), pp. 276-282).
The semiconductor laser optical amplifier is disadvantageous in that it exhibits a larger coupling loss with optical fibers than the rare earth element-doped optical fiber amplifier and that it is subjected to a large loss when current injection is stopped. That us, the rare earth element-doped optical fiber can pass a signal beam to some extent even if the excited beam source, which is a source for amplification, is isolated, allowing the insertion loss to be maintained within 5-dB or so as long as the amplifier is well designed. In contrast thereto, the semiconductor laser optical amplifier exhibits a larger coupling loss with fibers, with an average insertion loss of 10 dB or so including an internal loss of the amplifier. The insertion loss is increased by 5 to 10 dB, resulting in a total of 15 to 20 dB when the amplifier operation is stopped.
For example, if the internal gain is 23 dB, the effective gain is 13 dB (23-10) (a gain of 20) in a normal condition, while the effective gain is -15 dB (1/31.6) in an abnormal condition, causing the output to attenuate by 38 dB (1/6300) during malfunction of link amplifier compared to that in the normal condition.
For the above reason, the semiconductor laser optical amplifier is disadvantageous in ensuring the communication path at the time of trouble, i.e., the semiconductor laser optical amplifier is unsatisfactory from the viewpoint of fail-safe function compared to the rare earth element-doped optical fiber amplifier, although the semiconductor laser amplifier is advantageous in terms of its element structure that is small and in terms of its flexible capabilities with an extensive range of amplifiable wavelengths and a wide band for selected wavelengths.
With the optical communication network shown in FIG. 15, an optical signal is transmitted in a direction indicated by an arrow A. For example, the optical link amplifier 30b failed, an output from the optical link amplifier 30b becomes 1/6300 its input. Once thus attenuated, terminal units subsequent to the optical link amplifier 30b, e.g., the terminal unit 3c, can no longer receive an optical signal of sufficient level, leading to a grave malfunction of the optical communication network.
While it is conceivable to increase an input gain on the part of the terminal unit, an excessive increase in gain degrades the signal-to-noise ratio and the waveform attributable to the normal signal being saturated. Thus, there is a limit in increasing the gain.
A wavelength multiplex light source is employed as a light emitting element in an optical communication network, and example of which is a wavelength multiplex laser oscillator.
To design wavelength multiplex systems, it has been proposed to combine a plurality of semiconductor lasers, each having a different light emitting wavelength, and an optically dividing and synthesizing means together. In such systems, the use of individual semiconductor lasers leads to the problem of increasing the number of assembling steps in proportion to an increase in the number of channels.
To overcome this problem, what has been reported is a monolithic integrated DFB (distributed feedback) laser array which integrally arranges a plurality of semiconductor lasers, each having a different light emitting wavelength, on a single substrate (e.g., see H. Okuda et al., Japanese Journal of Applied Physics, Vol. 23, No. 12, December, 1984, pp. L904-L906).
However, even if such a semiconductor laser array is employed, optical signals are emitted from light emitting spots that are different from one another, thus requiring separate means for synthesizing the light.
What has been proposed to improve this shortcoming is a wavelength multiplex light source, which can both wavelength multiplex and synthesize optical signals using a laser excited by an external resonator that combines a semiconductor laser array and a diffraction grating, the semiconductor laser array having a coating of a nonreflective film on a single light exit end surface (see Unexamined Japanese Patent Publication No. Sho. 62-229891).
Here, the principle of the oscillating of the wavelength multiplex oscillator is described as follows.
When current is first fed to the "semiconductor laser amplifier", the "semiconductor laser amplifier" emits "spontaneous emission light". This light, like the LED light, is incoherent light of a broad spectrum. Of the spontaneous emission light, the light of a specific wavelength is selectively picked up by a spectral optical system. The selected light is returned to the semiconductor laser amplifier by way of an optical route of "optical waveguide".fwdarw.reflecting means.fwdarw."optical waveguide".fwdarw.spectral optical system. Only the selected light of the specific wavelength is amplified and travels through "semiconductor laser amplifier".fwdarw.reflecting means.fwdarw."semiconductor laser amplifier".fwdarw.. . . . Finally, a laser oscillation takes place.
That is, a source of the oscillation is emitted from the "semiconductor laser amplifier" and applied to "optical waveguide" by the spectral optical system.
As shown in FIG. 18, the external resonator of such conventional example described in the above publication includes a collimator lens 51, a diffraction grating 52, and a highly reflective coating 53a formed on-an end surface of an optical fiber 53, and an external oscillator. Semiconductor lasers LD.sub.1 to LD.sub.3 formed on a semiconductor laser array 54 emit light at respectively corresponding predetermined wavelengths .lambda..sub.1 to .lambda..sub.3. Reference character 54a designates a anti-reflective coating provided on one light exit end surface of the semiconductor laser array 54. A laser beam from each of the lasers LD.sub.1 to LD.sub.3 is diffracted by the diffraction grating 52 via the collimator lens 51 and is thereafter reflected again by the highly reflective coating 53a provided on the end surface of the optical fiber 53 via the collimator lens 51. The reflected laser beams return to the lasers LD.sub.1 to LD.sub.3 following the reverse path. The laser beams returning to the respective lasers have wavelengths .lambda..sub.1 to .lambda..sub.3 satisfying both predetermined angles .theta..sub.1 to .theta..sub.3 corresponding to the lasers LD.sub.1 to LD.sub.3 and a diffractive condition d sin .theta..sub.i =n .lambda..sub.i (where d is the distance between lines of the diffraction grating, and n is an integer). Therefore, individual semiconductor lasers LD.sub.1 to LD.sub.3 come to emit light at the corresponding predetermined wavelengths .lambda..sub.1 to .lambda..sub.3. Further, the laser beams from the lasers LD.sub.1 to LD.sub.3 are all injected onto the single optical fiber 53, so that the configuration shown in FIG. 18 serves also as a synthesizer. Reference character L in FIG. 18 designates a wavelength multiplex laser beam; .theta..sub.0, an angle formed between the normal to the diffraction grating 52 and an optical path from the diffraction grating 52 toward the optical fiber 53.
In the wavelength multiplex system, it is desirable that the wavelength width of a laser be narrow to facilitate not only the separation of respective wavelengths but also the optical processing of the light beam of each wavelength. In the conventional art disclosed in the above Unexamined Japanese Patent Publication No. Sho. 62-229891, however, the oscillation wavelength of a laser is equal to a wavelength bandwidth (spectral bandwidth) proportional to the diameter of the cross section of an optical fiber, thereby imposing the problem that it is difficult to narrow the wavelength bandwidth due to physical restrictions in the diameter of the cross section of an optical fiber. In addition, there has existed the problem that the coupling efficiency with the optical fiber is low. These problems will now be described below.
The reason why the oscillation wavelength of a laser has a wavelength bandwidth that is proportional to the diameter of the cross section of an optical fiber is as follows. As shown in FIG. 18, since the end surface of the optical fiber 53 provided with the highly reflective coating 53a is in an optically conjugate relation with the respective lasers LD.sub.1 to LD.sub.3 with respect to the spectral system formed of the collimator lens 51 and the diffraction grating 52, the diffracting conditions differ from one position to another on the end surface of the optical fiber 53. For this reason, the laser beam whose wavelength bandwidth (spectral bandwidth) is proportional to the diameter of the cross section of the optical fiber 53 starts oscillating.
FIG. 19 is an enlarged diagram showing the highly reflective coating 53a side Of the optical fiber 53. If it is assumed that the diameter of the optical fiber 53 consisting of a core 53b and a clad 53c is 2r, then a laser beam whose center wavelength is .lambda..sub.0 oscillates at a wavelength bandwidth ranging from a wavelength (.lambda..sub.0 +.delta..multidot.r) to a wavelength (.lambda..sub.0 -.delta..multidot.r), depending on the diffracting condition at a position on the end surface, where 6 is the proportional coefficient, which is equivalent to a reciprocal of the focal distance of the collimator lens 51.
While it is conceivable to narrow the wavelength bandwidth by decreasing the diameter of the optical fiber 53, predetermined standards prescribed on optical fibers for communication use do not easily permit the wavelength bandwidth of an optical fiber to be narrowed by changing its cross sectional structure.
The reason why the coupling efficiency with the optical fiber is low will be described next. A cross sectional structure of an ordinary optical fiber is shown in FIG. 20(a) and FIG. 20(b). FIG. 20(a) shows the cross sectional structure of a multimode fiber, with the outer diameter 2R.sub.f of its clad 53c being 125 .mu.m and the diameter 2R.sub.m of its core 53b being 50 .mu.m. FIG. 20(b) shows the cross sectional structure of a single mode fiber, with the outer diameter 2R.sub.f of its clad 53c being 125 .mu.m and the diameter 2R.sub.s of its core 53b being 10 .mu.m. If such a wavelength multiplex light source as shown in FIG. 18 is configured using these optical fibers, an emission spectrum of the laser oscillator is as shown in FIG. 21. That is, the wavelength bandwidth of a laser beam to be coupled with the core of a multimode fiber whose core diameter is 2R.sub.m is .DELTA..lambda..sub.m, while the wavelength bandwidth of a laser beam to be coupled with the core of a single mode fiber whose core diameter is 2R.sub.s is .DELTA..lambda..sub.s.
In the case where the multimode fiber such as shown in FIG. 20(a) is used, the amount of light to be coupled with the core 53b of the fiber (a region shaded by slants in FIG. 21) is large, but the spectral bandwidth of a laser beam is wide. Conversely, in the single mode fiber such as shown in FIG. 20(b), the spectral bandwidth of a laser beam is narrow, but the amount of light to be coupled with the core 53b of a fiber (a region shaded by intersections in FIG. 21) becomes small. Therefore, the coupling efficiency with the optical fiber is particularly decreased when the single mode fiber is used.